This invention relates to liquid slurries for use in a high-precision finishing process on the surface of a magnetic hard disk substrate and, more particularly, to liquid slurries containing polyhedral monocrystalline alumina such that fine central line average surface roughness (Ra) and a high polishing capability can be obtained. This invention also relates to a method of producing a magnetic hard disk substrate with its surface textured by using such a liquid slurry.
With the recent development in the so-called high-tech industries centered around the electronics industries, the memory capacity of magnetic disks is becoming higher and there is an increasing demand for high precision in the finishing of disk substrate surfaces. If a magnetic head is stopped on a magnetic disk thus structured, however, the magnetic head may be adsorbed to the magnetic disk due to the water component or a lubricant adsorbed to the disk surface. In order to prevent such occurrence of adsorption, it has been known to carry out a texturing process to form fine concentric protrusions and indentations on the surface of magnetic hard disk substrates in the circumferential direction of the substrate. The texturing process is usually carried out by using a liquid slurry obtained by dispersing abrading particles, say, of white molten alumina in a liquid, or a polishing tape obtained by coating a base material, say, of polyester with abrading particles for grinding.
Prior art abrading particles are obtained by pulverizing a base substance. Prior art alumina particles for grinding are obtained, for example, by pulverizing electrically molten alumina. When pulverizing a base abrading material, however, it is difficult to control the size and shape of the fragments or the manner in which the splitting occurs. Abrading particles obtained simply by pulverizing are not uniform and there are big variations in their sizes and shapes. If a texturing process is carried out by using a liquid slurry containing large abrading particles, these large particles tend to abrade the substrate surface excessively deeply, resulting in protrusions.
Another problem is that abrading particles thus obtained have very sharp edges at irregularly formed corners. If the texturing is carried out with abrading particles with sharp edges, the substrate surface is also scratched more deeply than desired or protrusions are formed. Moreover, such sharp edges of the abrading particles are likely to break off during the texturing operation. The portions that have broken off may be stuck to the substrate surface or displaced so as to form grooves where they are not intended. There are even situations where the alumina particles themselves become buried into the substrate surface to produce protrusions.
As the recording density on the magnetic disk is increased, the height of the magnetic head over the magnetic disk must be reduced in order to improve the signal sensitivity at the time of recording and reproduction. If there are protrusions sufficiently high on the substrate surface, however, the magnetic head may collide with such a protrusion (an event referred to as the "head hit"). If the texturing is carried out intentionally insufficiently in order to prevent the generation of such tall protrusions, however, the magnetic disk will end up being too smooth on the surface and the adsorption to the head will result, as described above.
It is necessary to use smaller abrading particles for reducing the center line average roughness. But a sufficiently large polishing capability is difficult to obtain with abrading particles with a small average diameter. Two steps of polishing may therefore be necessary, the first with larger abrading particles for rough polishing and the second with smaller abrading particles for fine polishing.